Chlorine-containing resins, particularly poly(vinyl chloride) (“PVC”) polymers and copolymers, are unstable to heat and light. Exposure to heat and light can cause degradation of the polymers, resulting in changes in the physical properties thereof. This degradation is typically manifested by changes in color, and is particularly noticeable in unstabilized polymers, i.e., polymers that do not contain stabilizers. Degradation or discoloration during processing is particularly undesirable in clear or lightly colored plastics. Therefore, it is desirable to prevent or inhibit the discoloration of plastics during processing so as to achieve useful products free of discoloration.
In order to minimize the discoloration and deterioration of various halogen-containing polymers such as vinyl chloride polymers and copolymers, stabilizers have been incorporated into the polymers. Frequently, these stabilizers contain heavy metals such as lead and cadmium. Environmental concerns with existing stabilizers have stimulated interest in alternative stabilizers, including organic stabilizers. For example, U.S. Pat. No. 3,928,285 to Gough et al. describes a synergistic stabilizer composition comprising an organotin borate and an organic thiol. In U.S. Pat. No. 4,948,827, Christidis describes a thiophenol having utility as a stabilizer. European Patent Application No. EP 0 945 484 A1 relates to compositions comprising halogen-containing polymers such as PVC resins that are stabilized against heat by a synergistic combination of a free mercaptan and a metal-based stabilizer and/or a Lewis acid such as zinc chloride. JP 63241055 A2 describes the compound dipentaerythritol hexakis(3-mercaptopropionate), and a method for stabilizing PVC with said compound. Starnes et al. describe numerous organic thiol stabilizers for PVC in U.S. Pat. Nos. 6,667,357, 6,762,231, 6,747,081, and 6,927,247, and published U.S. Patent Application Nos. 20050049307 and 20040220310.
While substantial efforts have been made to develop organic stabilizers for PVC, such stabilizers have achieved little commercial success, primarily as a result of their inability to compete with traditional stabilizers on a cost/performance basis.